The present invention relates to an exposure apparatus for use in a photographic printer, and more specifically, to a technique of correcting variations in the intensity of exposure light caused by deterioration with time of the performance of a component or namely, PLZT element in a digital exposure device which performs the exposure to a dot pattern of light.
In a conventional digital exposure device provided with a PLZT shutter, a given exposure requirement for each dot is translated into an exposure control signal which is then fed to a corresponding PLZT element for optimum exposure action.
It is however known that the PLZT element after a long service is declined in the performance causing an improper exposure action. Since the deterioration with time is not uniform among a plurality of the PLZT elements, its correction will be difficult.
For overcoming such a disadvantage of the PLZT shutter that a desired degree of optical density is hardly reproduced due to the deterioration with time, systems have been developed in which each PLZT element is examined for a degree of deterioration and an exposure correction signal is produced according to the degree of deterioration and fed to the PLZT for correction.
Among of them is an exposure device in which a sensor assigned to each PLZT element of the PLZT shutter is responsive to an output of light from the PLZT for producing and delivering a detection signal which is then used to modify the exposure control signal for the PLZT element so that declination of the exposure action caused by the deterioration with time of the PLZT element can be prevented.
The drawback of such a conventional exposure device is that a considerable number of the sensors have to be mounted to their respective PLZT elements of the PLZT shutter. This causes the PLZT shutter to have a more complex construction and will increase the overall cost of the exposure device.
It is an object of the present invention to provide a method of detecting and correcting a declination in the performance of each PLZT element without changing the substantial arrangement of a known PLZT shutter and also, an apparatus embodying the method.